1. Field of the Invention
This invention generally relates to an apparatus and method for the fabrication of semiconductor devices and more particularly to a structure and method for the construction of multi-chip power semiconductor devices in a low-profile metal-ceramic-metal (LPMCM) package.
2. Description of the Prior Art
The growth of the electronics industry worldwide has caused a need for a great variety of semiconductor products. The packaging of the so-called semiconductor stack (hereinafter, simply "stack", defined as the produced or purchased semiconductor device of whatever desired function may be desired, without connections, protective packaging or heat-sinking structure) to produce finished devices to be used in electronics products has been a matter of great importance in the semiconductor industry. Considerations involved in the choice or design of packaging for the semiconductor device will include the number of stacks which will be required in the device, the number of connections to be applied thereto, the intended apparatus within which the package is to be used, heat dissipation requirements and the like.
It is also the practice, where a power semiconductor device is required, to place a plurality of stacks within a single package for the purpose, on the one hand, of improving device reliability and specifications and, on the other hand, to provide economies in construction of the overall electronics product by minimizing the number of discrete components which must be assembled.
For the semiconductor manufacturer, the question of fabrication yield is also of major concern. First, the stack fabrication process will invariably produce some imperfect or non-functional stacks. The percentage of stacks produced which are functionally acceptable is referred to as the yield. When a plurality of such stacks are included in a single semiconductor device, the yield of the manufactured device may be far less than the yield of the stack fabrication process. This is because the inclusion of only one or a few defective stacks within a device would render the entire device functionally unacceptable. Since the present state of semiconductor technology will generally provide extremely high yields in the stack fabrication process, particularly where the design of the stack is simple and contains relatively few components for relatively low current usage, the problem of reduced device yields has recently been mostly limited to the fabrication of power semiconductor devices where the number of identical, parallel-connected stacks in a single device will be particularly large. However, where chip complexity reduces stack fabrication yields, even a few stacks per device may result in unacceptably low device yields.
To avoid reduced device fabrication yields a so-called (n-x) or (n-1) design philosophy has often been adopted. This means that in a device which contains n stacks of a similar type and similarly connected, x (typically 1) of the provided stacks may be defective within the performance specifications of the device even though such a defective stack or stacks may be required to be functionally disabled or removed from the circuit during fabrication. As an alternative or an addition to the (n-x) design rules, provision may also be made for the replacement of a defective stack within the device. Such an arrangement is shown for a single stack device in IBM Technical Disclosure Bulletin, Vol. 29, No. 3, August 1986, by J. A. Miraglia and J. H. Spreen, pp. 1071-1072.
When stacks of relatively simple design are used in power semiconductor devices, however, the cost of assembling a large number of stacks and other parts tends to increase costs and tending to defeat the potential efficiencies to be obtained in the assembly of a smaller number of devices in a particular product, as indicated above. Therefore, a need exists to simplify the construction of semiconductor devices having a large number of semiconductor stacks included therein.
In addition to the provision of a large plurality of parallel-connected stacks in the same semiconductor device, many special purpose devices require that the semiconductor stacks be accurately arrayed, Such applications include particle detectors, radiation sensors, microwave power receiving antennas and the like. Also, as miniaturization of power devices has increased, the arraying of the stacks has also become important to accommodates the increased criticality of heat dissipation from each respective stack and the heat dissipation and distribution throughout the multi-stack device.